The ongoing delay in the F-35 Joint Strike Fighter’s latest upgrades was one factor in the U.S. Air Force’s recent decision to purchase fewer jets in fiscal 2025.

The service announced Monday it plans to buy 42 F-35As from Lockheed Martin in 2025, six fewer than it originally anticipated. In a news conference to discuss the 2025 budget, top Air Force official Kristyn Jones told reporters that issues with the upgrades known as Technology Refresh 3 played a part in that decision, along with overall budgetary limitations.

“We want the planes that we want, and TR-3 [and] Block 4 capabilities have been delayed,” said Jones, who is performing the duties of undersecretary of the Air Force. “Our approach is to minimize the impact of that by procuring fewer of those” over the next few years, before ramping F-35 purchases back up to 48 in fiscal 2029.

TR-3 is the term for a series of hardware and software improvements to the F-35 that include better displays, computer memory and processing power.

The fighter needs the TR-3 improvements in place before a broader series of upgrades known as Block 4 can be rolled out. Block 4 is expected to allow the F-35 to carry more and better weapons, improve its target recognition, and upgrade its electronic warfare capabilities, among other changes.

But TR-3 is already almost a year behind schedule, and is not expected to be ready for months more, largely due to software and integration problems.

The delays have prompted the Pentagon to stop accepting deliveries of newly-built F-35s until TR-3 is ready. Lockheed Martin finished building the first F-35s that were to have the TR-3 improvements in July 2023. But because the software wasn’t ready, the Defense Department couldn’t conduct the check flights required to accept delivery.

F-35s have since been stored at Lockheed Martin facilities, particularly its main factory in Fort Worth, Texas, while work has continued on TR-3.

“TR-3 remains our top priority,” Lockheed Martin said in a statement to Defense News. “We expect to begin delivering TR-3 jets in the third quarter [of 2024] and are committed to providing unrivaled, advanced capabilities in support of our customers’ missions.”

Jones told reporters that the Air Force is still planning to buy 1,763 F-35s in all. The service was originally expecting to buy 48 F-35s annually over the next several years, but is now planning to also buy 42 in fiscal 2026, and 47 each in fiscal 2027 and 2028.

“We haven’t changed the total program of record,” Jones said. “But given the fiscal constraints this year, as well as the delays in getting the capabilities that we need, we rebased the program. But we haven’t cut off the total numbers.”

Lockheed said the company is continuing laboratory and flight tests of TR-3, while it keeps building F-35s. The company said flight tests now underway show improvements in the jet’s systems stability, as well as its video and weapons capability. The TR-3 configuration that it expects to be in place for delivery is also undergoing lab testing, Lockheed said, with improvements expected in system stability, its radar, and video capability.

Rep. Rob Wittman, R-Va., chairman of the House Armed Services subcommittee on tactical air and land forces, has grown increasingly displeased with the ongoing — and worsening — delays in the TR-3 effort and F-35 deliveries.

Having potentially dozens of F-35s unfinished for delivery “should be deeply disturbing to everyone across the enterprise, especially since the Air Force’s plan is to retire current fighter jet inventory,” Wittman told reporters March 7 at the McAleese Defense Conference in Washington.

Wittman is also concerned that jets might not be able to fly combat missions right away even when they’re delivered, if the F-35 program installs a partial version of the software as an interim fix. And he reiterated his concern that TR-3 delays will have a cascading effect that in turn holds up Block 4 upgrades in years to come.

“This has to be fixed in the long term,” Wittman said.